1. Field of the Invention
The present invention relates to an inverter control circuit and more particularly to an inverter control circuit which employs a ROM (Read Only Memory) or the like, unused storage regions thereof being utilized for storing voltage patterns dedicated for stopping an induction motor thereby for controlling number of revolutions of the induction motor.
2. Description of the Prior Art
As an inverter control circuit of the described type, there has been proposed a circuit as shown in FIG. 1. Referring to FIG. 1, reference numeral 1 denotes a voltage command generator outputting a six-bit signal, 2 denotes a frequency command generator outputting a six-bit signal, both thereof in general being functionally included in a microcomputer 6, 3 denotes a storage device, hereafter to be called a ROM, having 4096 8-bit words of storage capacity, 4 denotes a data selector, and 5 denotes a ring counter. In providing three-phase a.c. control signals, the circuit of FIG. 1 is adapted such that switching devices for two phases in the main circuit for the inverter apparatus may do switching operations in accordance with the voltage patterns stored in the ROM 3 while the remaining one phase may be either in conducting or nonconducting state. Further, combinations of conducting states of these three phases are adapted to be alternated at intervals of 60.degree. of electrical angles by means of the ring counter 5, whereby the three-phase a.c. control signals (U-phase, V-phase, and W-phase) are provided with the phase voltage patterns as shown in FIG. 3.
FIG. 2 indicates the manner of storing the voltage pattern data into the ROM 3 shown in FIG. 1. Since the voltage command is given in six bits, there are obtainable 64 ways at the maximum of the voltage patterns, and therefore, by making the range of the output frequencies of the inverter from 3 Hz to 60 Hz, it is possible to make the voltage patterns stored in increments of 1 Hz as shown in FIG. 2.
When an induction motor is slowed down and stopped by the inverter circuit as referred to in the foregoing controlled by the described circuit, the induction motor will be decelerated to the number of revolutions corresponding to 3 Hz and then allowed to stop after freely running for a while. Therefore, there was a defect that control precision for stopping was worsened especially in such a case that a load of larger inertia was connected to the induction motor.